The Siege of Utumno was a great battle where the Valar assaulted the greatest fortress of Melkor. Melkor ordered his Balrog generals to him as he stayed with a great host of evil beings fanatically loyal to him. The Valar proved mighty and powerful--in the end Melkor was tossed onto his face by Tulkas and bound by the mystical chain Angainor (forged by Aule).
It is conceivable that after the Valar besieged Angband, the forces at Angband under Sauron journeyed underground to Utumno to reinforce Melkor's forces.